alt Hacker NewsThis is called "purchasing power parity". There's an official index for it, as well as ad hoc measures like the Economist Big Mac Index.
To some extent it's circular: the US has a higher number of GDP because it spends more on healthcare. The broken leg version of the broken window fallacy.
This is an excellent point. Another comment pointed out that the gap in median salary is not as great as the gap in per capita GDP. Depending on the causes this and lower prices may mean Japanese are better off then Americans - e.g. if there is greater self-supply within households that would not be captured by GDP.